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    ## Middleware

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    Middleware (also called pre and post *hooks*) are functions which are passed
    control during execution of asynchronous functions. Middleware is specified
    on the schema level and is useful for writing [plugins](./plugins.html).

    <ul class="toc">
      <li><a href="#types-of-middleware">Types of Middleware</a></li>
      <li><a href="#pre">Pre</a></li>
      <li><a href="#error-handling">Errors in Pre Hooks</a></li>
      <li><a href="#post">Post</a></li>
      <li><a href="#post-async">Asynchronous Post Hooks</a></li>
      <li><a href="#defining">Define Middleware Before Compiling Models</a></li>
      <li><a href="#order">Save/Validate Hooks</a></li>
      <li><a href="#naming">Naming Conflicts</a></li>
      <li><a href="#notes">Notes on findAndUpdate() and Query Middleware</a></li>
      <li><a href="#error-handling-middleware">Error Handling Middleware</a></li>
      <li><a href="#aggregate">Aggregation Hooks</a></li>
      <li><a href="#synchronous">Synchronous Hooks</a></li>
    </ul>

    ### Types of Middleware

    Mongoose has 4 types
    of middleware: document middleware, model middleware, aggregate middleware, and query middleware.
    Document middleware is supported for the following document functions.
    In document middleware functions, `this` refers to the document.

    * [validate](/docs/api/document.html#document_Document-validate)
    * [save](/docs/api/model.html#model_Model-save)
    * [remove](/docs/api/model.html#model_Model-remove)
    * [updateOne](/docs/api/document.html#document_Document-updateOne)
    * [deleteOne](/docs/api/model.html#model_Model-deleteOne)
    * [init](/docs/api/document.html#document_Document-init) (note: init hooks are [synchronous](#synchronous))

    Query middleware is supported for the following Model and Query functions.
    In query middleware functions, `this` refers to the query.

    * [count](./api.html#query_Query-count)
    * [deleteMany](./api.html#query_Query-deleteMany)
    * [deleteOne](./api.html#query_Query-deleteOne)
    * [find](./api.html#query_Query-find)
    * [findOne](./api.html#query_Query-findOne)
    * [findOneAndDelete](./api.html#query_Query-findOneAndDelete)
    * [findOneAndRemove](./api.html#query_Query-findOneAndRemove)
    * [findOneAndUpdate](./api.html#query_Query-findOneAndUpdate)
    * [remove](./api.html#model_Model.remove)
    * [update](./api.html#query_Query-update)
    * [updateOne](./api.html#query_Query-updateOne)
    * [updateMany](./api.html#query_Query-updateMany)

    Aggregate middleware is for `MyModel.aggregate()`. Aggregate middleware
    executes when you call `exec()` on an aggregate object.
    In aggregate middleware, `this` refers to the [aggregation object](./api.html#model_Model.aggregate).

    * [aggregate](./api.html#model_Model.aggregate)

    Model middleware is supported for the following model functions.
    In model middleware functions, `this` refers to the model.

    * [insertMany](./api.html#model_Model.insertMany)

    All middleware types support pre and post hooks.
    How pre and post hooks work is described in more detail below.

    **Note:** If you specify `schema.pre('remove')`, Mongoose will register this
    middleware for [`doc.remove()`](./api.html#model_Model-remove) by default. If you
    want to your middleware to run on [`Query.remove()`](./api.html#query_Query-remove)
    use [`schema.pre('remove', { query: true, document: false }, fn)`](./api.html#schema_Schema-pre).

    **Note:** Unlike `schema.pre('remove')`, Mongoose registers `updateOne` and
    `deleteOne` middleware on `Query#updateOne()` and `Query#deleteOne()` by default.
    This means that both `doc.updateOne()` and `Model.updateOne()` trigger
    `updateOne` hooks, but `this` refers to a query, not a document. To register
    `updateOne` or `deleteOne` middleware as document middleware, use
    `schema.pre('updateOne', { document: true, query: false })`.

    **Note:** The [`create()`](./api.html#model_Model.create) function fires `save()` hooks.

    <h3 id="pre"><a href="#pre">Pre</a></h3>

    Pre middleware functions are executed one after another, when each
    middleware calls `next`.

    ```javascript
    var schema = new Schema(..);
    schema.pre('save', function(next) {
      // do stuff
      next();
    });
    ```

    In [mongoose 5.x](http://thecodebarbarian.com/introducing-mongoose-5.html#promises-and-async-await-with-middleware), instead of calling `next()` manually, you can use a
    function that returns a promise. In particular, you can use [`async/await`](http://thecodebarbarian.com/common-async-await-design-patterns-in-node.js.html).

    ```javascript
    schema.pre('save', function() {
      return doStuff().
        then(() => doMoreStuff());
    });

    // Or, in Node.js >= 7.6.0:
    schema.pre('save', async function() {
      await doStuff();
      await doMoreStuff();
    });
    ```

    If you use `next()`, the `next()` call does **not** stop the rest of the code in your middleware function from executing. Use
    [the early `return` pattern](https://www.bennadel.com/blog/2323-use-a-return-statement-when-invoking-callbacks-especially-in-a-guard-statement.htm)
    to prevent the rest of your middleware function from running when you call `next()`.

    ```javascript
    var schema = new Schema(..);
    schema.pre('save', function(next) {
      if (foo()) {
        console.log('calling next!');
        // `return next();` will make sure the rest of this function doesn't run
        /*return*/ next();
      }
      // Unless you comment out the `return` above, 'after next' will print
      console.log('after next');
    });
    ```

    <h4 id="use-cases">Use Cases</h4>

    Middleware are useful for atomizing model logic. Here are some other ideas:

    * complex validation
    * removing dependent documents (removing a user removes all his blogposts)
    * asynchronous defaults
    * asynchronous tasks that a certain action triggers

    <h4 id="error-handling">Errors in Pre Hooks</h4>

    If any pre hook errors out, mongoose will not execute subsequent middleware
    or the hooked function. Mongoose will instead pass an error to the callback
    and/or reject the returned promise. There are several ways to report an
    error in middleware:

    ```javascript
    schema.pre('save', function(next) {
      const err = new Error('something went wrong');
      // If you call `next()` with an argument, that argument is assumed to be
      // an error.
      next(err);
    });

    schema.pre('save', function() {
      // You can also return a promise that rejects
      return new Promise((resolve, reject) => {
        reject(new Error('something went wrong'));
      });
    });

    schema.pre('save', function() {
      // You can also throw a synchronous error
      throw new Error('something went wrong');
    });

    schema.pre('save', async function() {
      await Promise.resolve();
      // You can also throw an error in an `async` function
      throw new Error('something went wrong');
    });

    // later...

    // Changes will not be persisted to MongoDB because a pre hook errored out
    myDoc.save(function(err) {
      console.log(err.message); // something went wrong
    });
    ```

    Calling `next()` multiple times is a no-op. If you call `next()` with an
    error `err1` and then throw an error `err2`, mongoose will report `err1`.

    <h3 id="post"><a href="#post">Post middleware</a></h3>

    [post](/docs/api.html#schema_Schema-post) middleware are executed _after_
    the hooked method and all of its `pre` middleware have completed.

    ```javascript
    schema.post('init', function(doc) {
      console.log('%s has been initialized from the db', doc._id);
    });
    schema.post('validate', function(doc) {
      console.log('%s has been validated (but not saved yet)', doc._id);
    });
    schema.post('save', function(doc) {
      console.log('%s has been saved', doc._id);
    });
    schema.post('remove', function(doc) {
      console.log('%s has been removed', doc._id);
    });
    ```

    <h3 id="post-async"><a href="#post-async">Asynchronous Post Hooks</a></h3>

    If your post hook function takes at least 2 parameters, mongoose will
    assume the second parameter is a `next()` function that you will call to
    trigger the next middleware in the sequence.

    ```javascript
    // Takes 2 parameters: this is an asynchronous post hook
    schema.post('save', function(doc, next) {
      setTimeout(function() {
        console.log('post1');
        // Kick off the second post hook
        next();
      }, 10);
    });

    // Will not execute until the first middleware calls `next()`
    schema.post('save', function(doc, next) {
      console.log('post2');
      next();
    });
    ```

    <h3 id="defining"><a href="#defining">Define Middleware Before Compiling Models</a></h3>

    Calling `pre()` or `post()` after [compiling a model](/docs/models.html#compiling)
    does **not** work in Mongoose in general. For example, the below `pre('save')`
    middleware will not fire.

    ```javascript
    const schema = new mongoose.Schema({ name: String });

    // Compile a model from the schema
    const User = mongoose.model('User', schema);

    // Mongoose will **not** call the middleware function, because
    // this middleware was defined after the model was compiled
    schema.pre('save', () => console.log('Hello from pre save'));

    new User({ name: 'test' }).save();
    ```

    This means that you must add all middleware and [plugins](/docs/plugins.html)
    **before** calling [`mongoose.model()](/docs/api/mongoose.html#mongoose_Mongoose-model).
    The below script will print out "Hello from pre save":

    ```javascript
    const schema = new mongoose.Schema({ name: String });
    // Mongoose will call this middleware function, because this script adds
    // the middleware to the schema before compiling the model.
    schema.pre('save', () => console.log('Hello from pre save'));

    // Compile a model from the schema
    const User = mongoose.model('User', schema);

    new User({ name: 'test' }).save();
    ```

    As a consequence, be careful about exporting Mongoose models from the same
    file that you define your schema. If you choose to use this pattern, you
    must define [global plugins](/docs/api/mongoose.html#mongoose_Mongoose-plugin)
    **before** calling `require()` on your model file.

    ```javascript
    const schema = new mongoose.Schema({ name: String });

    // Once you `require()` this file, you can no longer add any middleware
    // to this schema.
    module.exports = mongoose.model('User', schema);
    ```

    <h3 id="order"><a href="#order">Save/Validate Hooks</a></h3>

    The `save()` function triggers `validate()` hooks, because mongoose
    has a built-in `pre('save')` hook that calls `validate()`. This means
    that all `pre('validate')` and `post('validate')` hooks get called
    **before** any `pre('save')` hooks.

    ```javascript
    schema.pre('validate', function() {
      console.log('this gets printed first');
    });
    schema.post('validate', function() {
      console.log('this gets printed second');
    });
    schema.pre('save', function() {
      console.log('this gets printed third');
    });
    schema.post('save', function() {
      console.log('this gets printed fourth');
    });
    ```

    <h3 id="naming"><a href="#naming">Naming Conflicts</a></h3>

    Mongoose has both query and document hooks for `remove()`.

    ```javascript
    schema.pre('remove', function() { console.log('Removing!'); });

    // Prints "Removing!"
    doc.remove();

    // Does **not** print "Removing!". Query middleware for `remove` is not
    // executed by default.
    Model.remove();
    ```

    You can pass options to [`Schema.pre()`](/docs/api.html#schema_Schema-pre)
    and [`Schema.post()`](/docs/api.html#schema_Schema-post) to switch whether
    Mongoose calls your `remove()` hook for [`Document.remove()`](/docs/api.html#model_Model-remove)
    or [`Model.remove()`](/docs/api.html#model_Model.remove):

    ```javascript
    // Only document middleware
    schema.pre('remove', { document: true }, function() {
      console.log('Removing doc!');
    });

    // Only query middleware. This will get called when you do `Model.remove()`
    // but not `doc.remove()`.
    schema.pre('remove', { query: true }, function() {
      console.log('Removing!');
    });
    ```

    <h3 id="notes"><a href="#notes">Notes on findAndUpdate() and Query Middleware</a></h3>

    Pre and post `save()` hooks are **not** executed on `update()`,
    `findOneAndUpdate()`, etc. You can see a more detailed discussion why in
    [this GitHub issue](http://github.com/Automattic/mongoose/issues/964).
    Mongoose 4.0 introduced distinct hooks for these functions.

    ```javascript
    schema.pre('find', function() {
      console.log(this instanceof mongoose.Query); // true
      this.start = Date.now();
    });

    schema.post('find', function(result) {
      console.log(this instanceof mongoose.Query); // true
      // prints returned documents
      console.log('find() returned ' + JSON.stringify(result));
      // prints number of milliseconds the query took
      console.log('find() took ' + (Date.now() - this.start) + ' millis');
    });
    ```

    Query middleware differs from document middleware in a subtle but
    important way: in document middleware, `this` refers to the document
    being updated. In query middleware, mongoose doesn't necessarily have
    a reference to the document being updated, so `this` refers to the
    **query** object rather than the document being updated.

    For instance, if you wanted to add an `updatedAt` timestamp to every
    `updateOne()` call, you would use the following pre hook.

    ```javascript
    schema.pre('updateOne', function() {
      this.set({ updatedAt: new Date() });
    });
    ```

    You **cannot** access the document being updated in `pre('updateOne')` or
    `pre('findOneAndUpdate')` query middleware. If you need to access the document
    that will be updated, you need to execute an explicit query for the document.

    ```javascript
    schema.pre('findOneAndUpdate', async function() {
      const docToUpdate = await this.model.findOne(this.getQuery());
      console.log(docToUpdate); // The document that `findOneAndUpdate()` will modify
    });
    ```

    However, if you define `pre('updateOne')` document middleware,
    `this` will be the document being updated. That's because `pre('updateOne')`
    document middleware hooks into [`Document#updateOne()`](/docs/api/document.html#document_Document-updateOne)
    rather than `Query#updateOne()`.

    ```javascript
    schema.pre('updateOne', { document: true, query: false }, function() {
      console.log('Updating');
    });
    const Model = mongoose.model('Test', schema);

    const doc = new Model();
    await doc.updateOne({ $set: { name: 'test' } }); // Prints "Updating"

    // Doesn't print "Updating", because `Query#updateOne()` doesn't fire
    // document middleware.
    await Model.updateOne({}, { $set: { name: 'test' } });
    ```

    <h3 id="error-handling-middleware"><a href="#error-handling-middleware">Error Handling Middleware</a></h3>

    _New in 4.5.0_

    Middleware execution normally stops the first time a piece of middleware
    calls `next()` with an error. However, there is a special kind of post
    middleware called "error handling middleware" that executes specifically
    when an error occurs. Error handling middleware is useful for reporting
    errors and making error messages more readable.

    Error handling middleware is defined as middleware that takes one extra
    parameter: the 'error' that occurred as the first parameter to the function.
    Error handling middleware can then transform the error however you want.

    ```javascript
    var schema = new Schema({
      name: {
        type: String,
        // Will trigger a MongoError with code 11000 when
        // you save a duplicate
        unique: true
      }
    });

    // Handler **must** take 3 parameters: the error that occurred, the document
    // in question, and the `next()` function
    schema.post('save', function(error, doc, next) {
      if (error.name === 'MongoError' && error.code === 11000) {
        next(new Error('There was a duplicate key error'));
      } else {
        next();
      }
    });

    // Will trigger the `post('save')` error handler
    Person.create([{ name: 'Axl Rose' }, { name: 'Axl Rose' }]);
    ```

    Error handling middleware also works with query middleware. You can
    also define a post `update()` hook that will catch MongoDB duplicate key
    errors.

    ```javascript
    // The same E11000 error can occur when you call `update()`
    // This function **must** take 3 parameters. If you use the
    // `passRawResult` function, this function **must** take 4
    // parameters
    schema.post('update', function(error, res, next) {
      if (error.name === 'MongoError' && error.code === 11000) {
        next(new Error('There was a duplicate key error'));
      } else {
        next(); // The `update()` call will still error out.
      }
    });

    var people = [{ name: 'Axl Rose' }, { name: 'Slash' }];
    Person.create(people, function(error) {
      Person.update({ name: 'Slash' }, { $set: { name: 'Axl Rose' } }, function(error) {
        // `error.message` will be "There was a duplicate key error"
      });
    });
    ```

    Error handling middleware can transform an error, but it can't remove the
    error. Even if you call `next()` with no error as shown above, the
    function call will still error out.

    <h3 id="aggregate"><a href="#aggregate">Aggregation Hooks</a></h3>

    You can also define hooks for the [`Model.aggregate()` function](api.html#model_Model.aggregate).
    In aggregation middleware functions, `this` refers to the [Mongoose `Aggregate` object](api.html#Aggregate).
    For example, suppose you're implementing soft deletes on a `Customer` model
    by adding an `isDeleted` property. To make sure `aggregate()` calls only look
    at customers that aren't soft deleted, you can use the below middleware to
    add a [`$match` stage](api.html#aggregate_Aggregate-match) to the beginning
    of each [aggregation pipeline](https://docs.mongodb.com/manual/core/aggregation-pipeline/).

    ```javascript
    customerSchema.pre('aggregate', function() {
      // Add a $match state to the beginning of each pipeline.
      this.pipeline().unshift({ $match: { isDeleted: { $ne: true } } });
    });
    ```

    The [`Aggregate#pipeline()` function](api.html#aggregate_Aggregate-pipeline)
    lets you access the MongoDB aggregation pipeline that Mongoose will send to
    the MongoDB server. It is useful for adding stages to the beginning of the
    pipeline from middleware.

    <h3 id="synchronous"><a href="#synchronous">Synchronous Hooks</a></h3>

    Certain Mongoose hooks are synchronous, which means they do **not** support
    functions that return promises or receive a `next()` callback. Currently,
    only `init` hooks are synchronous, because the [`init()` function](./api.html#document_Document-init)
    is synchronous. Below is an example of using pre and post init hooks.

    ```javascript
    [require:post init hooks.*success]
    ```

    To report an error in an init hook, you must throw a **synchronous** error.
    Unlike all other middleware, init middleware does **not** handle promise
    rejections.

    ```javascript
    [require:post init hooks.*error]
    ```

    <h3 id="next">Next Up</h3>

    Now that we've covered middleware, let's take a look at Mongoose's approach
    to faking JOINs with its query [population](/docs/populate.html) helper.
